Self-tuning controller that extracts process model characteristics

• US 5,394,322 A
• Filed: 07/22/1993
• Issued: 02/28/1995
• Est. Priority Date: 07/16/1990
• Status: Expired due to Term

• Current Assignee: Invensys Systems Incorporated (Invensys PLC)
• Original Assignee: The Foxboro Company (Schneider Electric SE)
• Inventors: Hansen, Peter D.
• Primary Examiner(s): Envall, Jr., Roy N.
• Assistant Examiner(s): BROWN, THOMAS
• Application Number:
  US08/096,600
• Time in Patent Office: 586 Days
• Field of Search: 364/148-152, 364/160-165, 364/157, 318/561
• US Class Current: 700/37
• CPC Class Codes: G05B 13/045 using a perturbation signal

Abstract

An apparatus and method for process control that extracts information from a process for developing a model of the process that is used to design system control. The apparatus includes means for selecting a process model form that has two or more selectable parameters. The apparatus also includes means for deliberately disturbing a process that is in open-loop operation and that is in a substantially settled state and further includes means for measuring the process response. The apparatus selects parameters for the process model form according to a function of the measured process response. In this way a complete model of the process is identified. The apparatus self-tunes by directly calculating new control parameters according to a function of the identified open-loop process model and a preselected target behavior.

Claims

1. Process control apparatus for attaining a selected closed-loop response of a control system, said apparatus comprising

an outer-control loop having

monitoring means, coupled to a controlled variable signal representative of a first characteristic of a process having an open-loop response, and to an operation control signal representative of a preselected value for said first characteristic of said process, said monitoring means for generating a difference signal representative of the difference between a signal characteristic of said controlled variable signal and said operation control signal, and

an outer-control element coupled to said monitoring means and having means for storing at least one control parameter being representative of a control parameter of said process control apparatus, and having means for generating an outer control signal as a function of said control parameter and of said difference signal, for manipulating said first characteristic of said process toward said preselected value,

an inner-control loop having

an inner-control element coupled to said controlled variable signal and having means for storing one or more control parameters representative of control parameters of said process control apparatus and having means for generating an inner control signal as a function of a said control parameter and of said controlled variable signal, for manipulating said first characteristic of said process toward said preselected value, and

summation means, coupled to said outer-control element and to said inner control element, for summing said inner-control signal and said outer-control signal to generate a manipulated variable signal for manipulating said first characteristic of said process toward said preselected value, and

automatic tuning means for modifying said control parameters, said tuning means having means for storing at least one preselected closed-loop transfer function of said outer-control loop, and having means for storing at least one preselected closed-loop transfer function for said inner-control loop, and having means for storing at least one process model representative of a said open-loop process response, and having means for generating said control parameters as a function of said process model, and said preselected transfer functions, for tuning said inner-control loop and said outer-control to achieve substantially said selected closed-loop response.

2. Process control apparatus according to claim 1, having means for selecting one of said stored transfer functions according to a user-selected design criteria and wherein said design criteria for said first inner-control loop, represents the substantial elimination of derivative action for an open-loop process response representative of a pure delay.

3. Process control apparatus according to claim 1, having means for selecting one of said stored transfer functions according to a user-selected design criteria and wherein said design criteria for said first outer-control loop, represents the substantial elimination of overshoot of said controlled variable signal.

4. Process control apparatus according to claim 1 wherein said means for storing preselected transfer functions includes means for storing at least one transfer function being representative of a pure delay.

5. Process control apparatus according to claim 1 wherein said means for storing preselected transfer functions includes means for storing at least one transfer function being representative of a butterworth filter.

6. Process control apparatus according to claim 1 wherein said means for storing preselected transfer functions includes means for storing at least one transfer function being representative of a plurality of equal lag delays.

7. Process control apparatus according to claim 6 wherein said tuning has means for storing at least four process parameters and has means for generating said one or more control parameters as a function of said four process parameters and said transfer function being representative of said plurality of equal lag delays.

8. Process control apparatus according to claim 1 wherein said control parameters include a first control parameter, D, being representative of the derivative time of a control system, and said tuning means includes means for adjusting said derivative time control parameter according to a predetermined ratio of said stored process model to adjust derivative action within the control system.

9. Process control apparatus according to claim 1 for use within a non-interacting control system wherein said control parameters include a first non-interacting control parameter, P, being representative of the proportional band of the control system, and a second non-interacting control parameter, I, being representative of the integral time of the control system, and a third non-interacting control parameter, D, being representative of the derivative time of the control system,

said tuning means further comprising

means for converting said proportional, said derivative and said integral non-interacting control parameters into interacting control parameters representative of a proportional, a derivative and an integral control parameter for a corresponding interacting control system.

10. Process control apparatus according to claim 1, further including filter means for filtering said operation control signal, said filter means having a time constant τ

_f being determined according to a known function of said at least one control parameters.

11. Process control apparatus according to claim 1, further including means for storing a control parameter, α

, being representative of the lead-lag ratio of said process control means and said tuning means for generating said one or more control parameters has means for generating said led-lag ratio as a function of said at least one stored control parameter.

12. Process control apparatus for attaining a selected closed-loop response of a control system, said apparatus comprising

process control means having means for monitoring a controlled variable signal being representative of a first characteristic of a process having an open-loop response,

means for monitoring an operation control signal of said control system, said operation control signal being representative of a preselected value for said first characteristic of said process,

means for storing at least one parameter, said parameter being representative of a control parameter of said control system,

means for generating a manipulated variable signal as a function of at least said one control parameter, of said controlled variable signal and of said operation control signal, for manipulating said first characteristic toward said preselected value, and,

automatic tuning means for generating one or more control parameters, said tuning means includes

identification means for identifying a process model being representative of the open-loop process response, said process model having at least one variable parameter, said identification means having means for storing at least one process model, having means for selecting one of said stored process models and having means for selecting said at least one model parameter,

means for determining at least one weighting coefficient of a corresponding frequency component of said open-loop process response, said at least one weighting coefficient being determined according to a known function of said at least one model parameter,

means for storing at least one preselected closed loop system response, and,

means for generating said one or more control parameters as a function of said weighting coefficients, wherein said function is determined by a selected one of said stored system responses.

13. Process control apparatus according to claim 12, wherein said tuning means includes actuator means for actuating said open-loop process, said actuator means has means for storing at least one predetermined signal pattern, and means for generating said manipulated variable signal according to said predetermined signal pattern for application to said process to generate said controlled variable signal being representative of said open-loop process response to said predetermined signal pattern.

14. Process control apparatus according to claim 13, wherein said means for generating said manipulated variable signal includes

means for storing at least one predetermined threshold value being representative of a first characteristic of said controlled variable signal,

means for measuring said first characteristic of said controlled variable signal and means for measuring a second characteristic of said controlled variable signal, and,

wherein said means for generating said manipulated variable signal according to said predetermined signal pattern further includes means for generating said manipulated variable signal responsive to said first characteristic and said second characteristic of said controlled variable signal and said threshold value.

15. Process control apparatus according to claim 12, wherein said means for selecting one of said process models has

means for measuring at least a first and a second characteristic of said controlled variable signal, and,

means for determining from said first and said second characteristics, pattern features of said controlled variable signal being representative of one of said stored process models and being representative of said parameters of said process model.

16. Process control apparatus according to claim 15, wherein said means for determining pattern features of said controlled signal includes filter means for selecting predetermined frequencies of said controlled variable signal.

17. Process control apparatus according to claim 15, wherein said means for measuring a first characteristic and a second characteristic of said controlled variable signal has means for measuring the amplitude of said controlled variable signal.

18. Process control apparatus according to claim 15, wherein said means for measuring a first characteristic and a second characteristic of said controlled variable signal has means for measuring time intervals between preselected signal events.

19. Process control apparatus according to claim 18, wherein said preselected signal events include, the occurrence of local extrema, and the occurrence of zero-crossings.

20. Process control apparatus according to claim 15, wherein said means for measuring a first and a second characteristic of said controlled variable signal has means for measuring the amplitude of said controlled variable signal, and has means for storing at least one user-selected threshold value, each value being representative of a predetermined amplitude of said controlled variable signal.

21. Process control apparatus according to claim 20, wherein said means for measuring a first and a second characteristic of said controlled variable signal has means for measuring time intervals between preselected signal events, wherein said preselected signal events include, the occurrence of local extrema, the occurrence of zero-crossings and the occurrence of threshold crossings.

22. Process control apparatus for attaining a selected closed-loop response of a control system, said apparatus comprising

process control means for monitoring a controlled variable signal representative of a first characteristic of a process having an open-loop response,

means for monitoring an operation control signal of said control system, said operation control signal representative of a preselected value for said first characteristic of said process,

means for storing at least one control parameter representative of a control parameter of said process control apparatus,

means for generating a manipulated variable signal as a function of said control parameter, of said controlled variable signal and of said operation control signal, for manipulating said first characteristic toward said preselected value,

identification means for identifying a process model having at least one variable process parameter and being representative of said open-loop process response, including means for storing a first characteristic and a second characteristic of said process model, said first and said second characteristics being representative of a process type and being determined according to a ratio of said control parameters,

measurement means for measuring amplitude values of said controlled variable signal to characterize at least two pattern features representative of a first characteristic of the closed loop system,

means for generating said variable process parameters of said process model according to a known function of said first and said second process type characteristics and said first characteristic of said closed-loop system,

means for determining at least one weighting coefficient of a corresponding frequency component of said open-loop process response, said weighting coefficient being determined according to a known function of said model parameter,

means for storing at least one preselected closed loop system response, and,

automatic tuning means for generating said control parameters as a function of said weighting coefficients, wherein said function is determined according to a selected one of said stored closed loop system responses.

23. Process control apparatus according to claim 22, wherein said measurement means has means for measuring amplitude values being representative of said local extrema of said controlled variable signal.

24. Process control apparatus according to claim 22, wherein a first one of said two or more pattern features is representative of the period of said controlled variable signal and a second one of said at least two pattern features is representative of said attenuation of said controlled variable signal.